Jón Páll started his early career as a young bodybuilder and powerlifter. In 1984 Jón won the Icelandic bodybuilding title in the +90 kg. class. His achievements in powerlifting also include Icelandic records in the bench press (with 192.5 kg, 195 kg, 210 kg and 222.5 kg) and the squat (with 320.5 kg, 330 kg, 342.5 kg and 357.5 kg), but his best performances were usually in the deadlift event, in which he set the European record many times (with 350 kg, 352.5 kg, 360 kg, 362.5 kg and 370 kg) and multiple world records in strongman competition deadlift variations, such as the rectangular handled wheel and one handed deadlift.[4]

Jón Páll was invited to the World's Strongest Man competition for the first time in 1983, in which he came in second only to Geoff Capes. The following year at the young age of only 24, he managed to defeat Capes and secure the title. During the final armwrestling event, in which Jón Páll was up against him, Capes appeared to be winning, pulling Jón Páll's arm down convincingly, but sustained a muscle tear in his forearm as Jón Páll started to thrust his arm back. Right after winning the bout, Jón Páll shouted "The King, has lost his crown!" and won his first World's Strongest Man title in 1984. Although Jón Páll was closely defeated by Capes at the 1985 World's Strongest Man, he managed to regain the title in 1986. During the deadlift event at the 1985 World's Strongest Man competition, someone in the audience called him an Eskimo. Jón Páll shouted back: "I am not an Eskimo, I am a Viking!" and successfully lifted the 495 kg cart.

In 1986, Jón Páll first wrestled English Author and Guinness World Record Holder Brian Sterling-Vete in a demonstration match for the TV news and print media held at Finnur Karlsson’s gymnasium in Reykjavík, Iceland. Halfway through the match Jón and Sterling had plotted to surprise the audience with a supposed angry outburst leading to the two of them demonstrating their skills as martial artists. This very typical ‘showmanship’ became synonymous with both Jón Páll and Sterling.

In 1987, Jón Páll clashed with his arguably greatest rival - 3 times World's Strongest Man winner Bill Kazmaier of Auburn, Alabama, USA, who had not been invited to compete at World's Strongest Man again after winning the competition 3 times in a row from 1980 to 1982. Kazmaier boasted some of the heaviest powerlifting lifts of that time including world records in the bench press with 300 kg (661.4 lbs), deadlift with 402 kg (886.7 lbs) and total 1100 kg (2425.08 lbs) and had made his reputation in the 80's as "the strongest man who ever lived" by breaking numerous strongman world records of the 20th century.

At Pure Strength 1987, a competition held in place of the absent World's Strongest Man competition of that year on the grounds of Huntly Castle in Aberdeenshire, Scotland, Geoff Capes, Bill Kazmaier and Jón Páll matched up to crown the strongest man on the planet.[5] Jón Páll, being in the shape of his life, won the contest convincingly by winning 8 out of 10 events and even managed to beat Kazmaier, who was making his comeback into the strongman sport after having worked and travelled as a professional wrestler.[3] A famous quote, "there is no reason to be alive if you can't do deadlift" was shouted by Jón Páll when he won the deadlift event at this contest with a strongman world record lift of 523 kg off a rectangular handled bar from knee height (1153 lbs).

At the 1988 World's Strongest Man the two rivals clashed again. As expected Kazmaier dominated the static events while Jón Páll, who had shed body-weight to cater for all the dynamic tests of strength of World's Strongest Man instead of the latter statically orientated events of Pure Strength 1987, was often victorious in the more athletic, speed- and endurance-oriented events. Although Jón Páll was beaten by Kazmaier in the deadlift, log lift and sack race, he managed to win the "weight over the bar event" and the McGlashen Stones in the end to secure the overall victory and to become the World's Strongest Man for the third time equaling Kazmaier's record. After his victory Jón Páll said: "I may be the fastest strongman in the world, but I think Bill [Kazmaier] is the strongest on his feet."

After a disappointing third place at the 1989 World's Strongest Man, Jón Páll was able to win the competition, while injured, for a record breaking fourth time in 1990. O.D. Wilson, who was leading the competition with a comfortable 5½ points before the last event - a 200 m race with a 100 kg weight on the back - was a very heavy 400-pound man, who lacked the endurance and running speed to complete the course quickly enough and ended up losing by half a point to the much lighter and faster Jón Páll Sigmarsson.

The 1990 World's Strongest Man proved to be Jón Páll's last appearance on the World's Strongest Man stage, as he was surpassed as the Icelandic number one by fellow countryman Magnús Ver Magnússon, who won the Iceland's Strongest Man title in 1988 and 1989 and continued Sigmarsson's legacy by emulating his 4 World's Strongest Man title wins during the 90's.

Two other famous quotes of Jón Páll's are "I am in seventh heaven", which was his translation of an Icelandic saying, and "Ekkert mál fyrir Jón Pál", which translates as "No problem for Jón Páll". Jón Páll remained an icon for the Strongman sport. His charm and charisma off stage made him a crowd favourite and his supreme strength was manifested in his infamous and exuberant Viking chanted performances on stage. With four Worlds Strongest Man titles, Jón Páll Sigmarsson is one of the three most successful competitors in the history of the contest. He is only equaled by Magnús Ver Magnússon and Žydrūnas Savickas with four titles and surpassed by Mariusz Pudzianowski with five titles.

Jón Páll was also one of a few strongmen who possessed the genetics that allowed him to master all forms of strength. He was able to master the Olympic weightlifting lifts as well as exert tremendous static strength that allowed him to defeat some of the greatest powerlifters of all time. This combination of strength manifested well in World's Strongest Man, leaving no chinks in his armour. Coupled with strength training he also achieved feats in the Bodybuilding world. This included his famous bodybuilding and strongman wins in 1988, where he had to contrast his heavy strongman training and bulk of up to 140 kg with a cutting up phase to 110 kg for the bodybuilding contests. His weight remained consistently around 130 kg for the World Strongest Man contests, where versatility would be compromised by heavy bodyweights, a point emphasized by Ab Wolders of the Netherlands in 1989, a fellow competitor. Jón Páll weighed his heaviest during Pure Strength 1987 at a weight of 143 kg.

Jón Páll was described in his life documentary "Larger than Life" by one of his former rivals Geoff Capes as "a new kind of guy on the block" and by David Webster equally in describing him as a man who boasted pure static strength as well as versatile athletic strength in all its forms, a combination that lead to his supreme performances.

After battling with the injuries sustained during 1988, 1989, and 1990, which had affected his athleticism, his strength became progressively more static in his later years. Jon Pall was often challenged by fellow competitor Bill Kazmaier in feats of statically oriented events between contests because he believed the events were biased in Jón Páll's favour. This included Kaz challenging Jon Pall to a sledgehammer hold during Pure Strength 1987 where Jón Páll beat Kazmaier. Kazmaier had travelled to Nigeria for a strength exhibition with Douglas Edmunds to break the deadlift world record by deadlifting 414 kg ( 914 lbs ) with straps, Jon Pall then re-broke the record with a lift of 427.5 kg ( 942 lbs ) in his gym in Iceland in front of spectators in 1987 at his heaviest bodyweight in preparation for Pure Strength 1987.[7]

On 16 January 1993 Jón Páll Sigmarsson suffered a cardiac arrest. He was deadlifting in his gym, the Gym 80 in Iceland when the cardiac arrest occurred, caused, it is thought, by a traumatic aortic rupture which was related to a weakened heart, a problem that was known to exist in his family.[citation needed]

He also achieved world records in the Claymore Lift, McGlashen Stones and the Cannon Pull during Pure Strength 1987.[9] He was also the first man ever to load a 150 kg McGlashen stone[4] and got into the Guinness Book of Records for lifting the world's largest whiskey bottle.[4]